Polymer electrolyte fuel cells (PEFCs) have been developed for many applications, including low power applications that are now served by conventional batteries. PEFCs require a fuel supply, such as hydrogen or methanol, and an oxidant, which may be air. Early fuel cells required systems that provided fuel cell cooling and a pressurized and humidified air supply. These systems did not enhance the portability of PEFCs.
There are various portable power supplies, e.g., electrochemical batteries. However, batteries have a finite lifetime and cannot be recharged or regenerated in the field. Fuel cells can be readily resupplied with fuel and oxidant to provide a power supply for extended use. The usefulness of fuel cells could be greatly extended if a compact portable fuel cell was available.
The most desirable features of a portable power source are high power density, energy capacity, simple control system, convenient operation, low acoustic and thermal signatures, and ease of mass production. A passive air fuel cell system with hydrogen or methanol fuel can readily fulfill these requirements and offer significant advantage over the advanced batteries available today, especially in terms of the power density and energy capacity for long mission duration. The hydrogen/passive air fuel cell according to this invention is operated with ambient air naturally diffused to the fuel cell cathode. Since there are no pumps or other moving parts and energy consuming peripheral equipment involved in the system, system reliability and energy conversion efficiency are greatly enhanced. Also, because the air reactant transport to the electrode surface occurs by natural diffusion in the passive stack, an even reactant distribution among the cells can be more conveniently achieved than in a system using forced air feed.
Thus, for a passive air system, the control system can be greatly simplified or eliminated, and operation becomes more user friendly. Because of these merits, the passive air power systems have potential as portable power sources of choice for both military and commercial markets, especially for low power applications, where the overall system specific energy density requirement, as high as a few thousands watts per kg, does not permit the added weight of complicated auxiliary equipment and control systems. Indeed, the passive air fuel cell systems are much more like batteries in the sense of system simplicity and are more suitable for portable power applications.
U.S. Pat. No. 5,514,486 is directed to a passive (“air-breathing”) portable PEFC where hydrogen fuel is supplied through a central annulus and air is supplied through diffusion along a radially directed porous flow field about the periphery of the device. The porous flow field acts to retain water reaction products in the cell to maintain hydration of the polymer electrolyte and to affect cooling of the cell. A drawback in the annular design is the limitation in the size of electrode area. As the electrode area is increased, the portion of the peripheral area from which oxygen from air diffuses readily to the electrode becomes small and stack performance suffers. The annular design provides greater energy than a conventional NiCad battery of similar size, but does not deliver power levels as high as NiCad batteries.
The present invention provides a portable fuel cell stack that has a high power density and does not require auxiliary equipment for the supply of reactants, i.e., hydrogen or methanol and oxygen from air. A rectangular cell geometry and an open air flow field permit good access of air to the air cathode and easy release of water which is the fuel cell reaction product. By limiting the losses arising from air cathode polarization in a passive air fuel cell stack, a high stack volume power density and high energy conversion efficiency can be achieved. The demonstrated power and energy conversion efficiency of a non-optimized testing stack discussed herein are already attractive for a wide range of military and commercial applications.
Various features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.